While European researchers have previously found that tobacco smoke can damage sperm DNA, it seems that the fertility of smokers could be even harder hit by tobacco than previously thought. In a study presented at the American Society of Reproductive Medicine this week, researchers from the University at Buffalo (UB) said men who smoke cigarettes may experience a significant decline in their capacity to father a child due to less-than-healthy sperm.

The researchers, led by UB reproductive expert Lani Burkman, found that the sperm in two-thirds of the smokers in the study failed a special test that measures the ability of sperm to fertilize an egg. Burkman said that on average, the smokers showed a 75 percent decline in fertilizing capacity when compared to non-smokers.

Burkman explained that just like other cells in the body, human sperm recognize and respond to nicotine. "This happens because nicotine from tobacco mimics one of the most important neurochemicals produced in the body. In this new study, we examined whether sperm from chronic tobacco smokers are defective in binding to the zona, the cover surrounding an egg. Our results could mean that heavy smoking overloads the nicotine receptor in human sperm and in the testes, leading to a decline in fertilizing potential."

The subjects in the study - funded, surprisingly enough, by tobacco company Philip Morris - were men who reported smoking at least four cigarettes a day, every day, for more than two years. Their sperm function was compared to the sperm of non-smoking donors with normal fertilizing capacity. To do this, a test called the Hemizona Assay was used. This involved the researchers cutting an egg in half and placing one half with a smoker's sperm and the matching half with non-smoking sperm. After two to three hours, the researchers counted the number of sperm attached to each half of the egg. "To fail, the index must be less than 65, meaning that the smoker's sperm had less than 65 percent of the fertilizing capacity found in the [non-smoking] donor," explained Burkman. "An index below 36 identifies a severe loss in fertilizing capacity."

The results showed that the sperm from almost two-thirds of the smokers failed the test, while the remainder showed normal function. Almost all the smokers whose sperm failed the test had an index of 36 or less, with an average of 25. "None of these men had a zero fertilizing potential," cautioned Burkman, "but the results mean that their sperm had only 25 percent of the fertilizing function found in nonsmoking men. The data also showed that the men who failed were smoking about twice as many cigarettes per day, an average of 19 per day, compared to the smokers who passed the assay." The study, "clearly reveals a significant drop in fertility potential for men who are heavy tobacco smokers," she concluded.